photoelectron spectroscopy of mass-selected copper-water …

Laser Chem., Vol. 15, pp. 195-207Reprints available directly from the PublisherPhotocopying permitted by license only

(C) 1995 Harwood Academic Publishers GmbHPrinted in Malaysia

PHOTOELECTRON SPECTROSCOPY OFMASS-SELECTED COPPER-WATER CLUSTER

NEGATIVE IONS

FUMINORI MISAIZU, KEIZO TSUKAMATO, MASAOMI SANEKATA andKIYOKAZU FUKE

Institute for Molecular Science and The Graduate University for AdvancedStudies, Myodaiji, Okazaki 444, Japan

(Received 12 April, 1994)

Negative-ion photoelectron spectroscopy has been applied in order to obtain size dependent informationabout the electronic structure of clusters of metal atoms solvated with polar molecules. In the presentpaper we have investigated the photoelectron spectra of Cu-(H20), cluster ions with n 0-4 and alsothose of CUz-(H20), with n 0 and 1. In the spectra of Cu-(H20),,, the lowest energy bands were assignedto the electron detachment from the CuOH-(H20),_ which were produced in the source together withthe above cluster ions. The observed bands for Cu-(HzO)n were all assigned to the transitions to thestates originating in the ground 2S and first excited 2D states of the Cu atom. The stepwise hydrationfor Cu- and Cu2- was discussed from the observed spectral shifts.

KEY WORDS: Photoelectron spectroscopy, Clusters, Solvated metal atom, Copper, Hydration

1. INTRODUCTION

Electronic structure of the clusters with one metal atom solvated by polar moleculeshas been one of the central interests for many researchers for the purpose of unveilingthe microscopic aspect of the bulk electrolyte solution. In particular, the electronicstates having an ion-pair character in the clusters with an alkali metal atom or analkaline-earth metal ion ligated by polar solvent molecules such as H20 and NH3have been investigated both experimentally1- and theoretically.-9 Characterizationof the ion-pair state in which an electron of the metal atom is delocalized over thesolvent molecules with increasing number of ligands may provide microscopic in-formation on the solvation process of a metal atom/ion in bulk liquid. In the caseof alkaline-earth metal ion-solvent systems, stabilization of the ion-pair state as afunction of the cluster size was examined by photodissociation spectroscopy. -7 Espe-cially for Sr+(NH3)n, Farrar and his coworkers discussed the possibility of aRydberg-type ion-pair state from the large redshift of the absorption band. -4 Thephotodissociation of Mg+(HzO)n clusters was also investigated by the presentauthors,5-7 and some clues of the ion-pair state have been obtained for this system.

195

196 FUMINORI MISAIZU et al.

As for the neutral alkali atom-ligand systems, the information about the excitedelectronic states with charge-transfer character was obtained from the ionizationpotential (IP) measurement only indirectly so far. IPs of Na(HzO)n and Na(NH3),were measured by Hertel and his coworkers8,9 and those of CS(HzO)n and Cs(NH3)nwere determined by the authors, l Both of the Na and Cs systems were found tohave similar features: For the metal-H20 systems, IP decreases monotonously withincreasing n up to 4, however, it becomes constant for n > 5 at a value whichcoincides with the photoelectric threshold of bulk ice. In contrast, the IP of themetal-NH3 system decreases linearly with increasing cluster radius for n up to 30and the limiting value again coincides with the estimated IP of an excess electronsolvated in liquid ammonia. These results can be explained by the stabilization ofthe ion-pair state with increasing number of water molecules" Such state is consideredto become the ground state for (Na, CS)-(HzO)n clusters with n 4, whereas moreligands are necessary to stabilize the state for (Na, Cs)-(NH3), systems. However,an alternative interpretation for these results has been proposed theoretically.9 It isinformative to obtain more decisive data about the electronic structures of the sol-vated alkali metal clusters.

Photoelectron spectroscopy (PES) of negative ion clusters is one of the mostpowerful method to get information not only on the stability of the negative ionsbut also on the electronic structure of the neutral clusters. This method was appliedto various gas phase clusters in the past decade. Recently, Cheshnovsky and hiscoworkers reported the photoelectron spectra of the cluster negative ions of halogenatom solvated with H20, CO2, and NH3 molecules.2-22 In the photoelectron spectrumof CI-(NH3), they assigned a broad band peaked at 6.45 eV binding energy to be thetransition to a charge-transfer (CT) state, C1-NH3+.22 As far as we know, this is theonly species that the CT state was observed in the photoelectron spectrum, thoughelectron transfers from the ligand to the solute atom in this system.

In this paper we report the PES results of the negative ions of copper-waterclusters. The coinage metal atoms, Cu, Ag, and Au, have electronic configurationsof 2S ((n-1)dns) in its ground state and thus have electronic characters analogousto the alkali metal atoms. They also have positive electron affinities because of thestabilization of the negative ions as a result of the ns shell closing with addition ofan excess electron. Therefore we investigated the Cu-water systems for the reasonsboth of testing the performance of the newly developed photoelectron spectrometerand of investigating the metal-water systems analogous to the hydrated alkali metalclusters. We have obtained the photoelectron spectra of Cu-(H20). (rt 0 4) andCUz-(HzO)n (n 0 and 1). Some of the lower-energy bands observed in the spectraare assigned to the electron detachment from the CuOH-(H20), ions produced simul-taneously with the above ions. The observed bands for Cu-(HzO)n are all assignedto the transitions to the neutral states derived from the 2S(3d4s) and 2D(3d94s2)levels of the free Cu atom. The successive hydration energies of Cu-(HzO)n in thesestates are found to be almost constant for the n range examined in this study andare discussed in line with the PES results of the hydrated halogen atom clusters.Stabilization of each band observed in the photoelectron spectrum of Cu2-by hydra-tion is also discussed in comparison with those of the hydrated Cu- clusters.

MASS-SELECTED COPPER-WATER CLUSTER NEGATIVE IONS 197

2. EXPERIMENTAL

Three-stage differentially evacuated chambers were constructed for the photoelectronspectroscopy of the negative ions. They consist of a negative ion beam source, time-of-flight (TOF) mass spectrometer, and a magnetic-bottle photoelectron spectrometeras shown in Fig. 1. Negatively-charged ions of copper-water clusters were producedby a laser vaporization. The details of the cluster source were similar to those ofthe cluster positive ions which were presented in the previous papers.5,6 Second har-monic of a Nd:YAG laser (Continuum, YG661) was focused on to the copper rod(t3 mm) which was rotating and translating in an aluminum block. Helium gas of2 atm mixed with water vapor at room temperature was expanded through the blockfrom a pulsed valve (General Valve, Series 9 or Model PSV, R.M. Jordan Co.).The negative ions directly produced in the source were accelerated to ca. 800 eV ina Wiley-McLaren type TOF mass spectrometer23 by pulsed electric fields and weremass-separated in the mass spectrometer. The TOF mass spectrum of the nascently

ID"HFM

Photoelectron Spectrometer

PS and DL

L2

D2 .,/MG

TOF Mass SpectrometerLI

DI

Cluster Beam Source

AP V

Figure 1 Schematic drawings of the experimental apparatus. PV: pulsed valve, AP: acceleration plates,D1, D2: deflectors, L1, L2: einzel lenses, MG: pulsed mass gate, PS: potential switch, DL: decelerationlenses, HFM: high field magnet, GS: guiding solenoid, ID: ion detector, ED: electron detector.


produced negative ions was recorded by using dual microchannel plates (Hamamatsu,F1551-23S) placed at the end of the chamber. For the photoelectron kinetic energymeasurement, negative ions with a given mass-to-charge ratio were selected with apulsed mass gate after flying 50 cm, and were decelerated to several tens of eV witha pulsed potential switching method. Decelerated ions were irradiated with a detach-ment laser at right angles in the center of the third chamber. The magnetic bottlephotoelectron spectrometer was used to measure the kinetic energy of the detachedelectron: it has an advantage to collect almost all the electrons emitted over 4rrsteradian with a magnetic field produced by a combination of a strong field magnetnear the detachment zone and a weak field solenoid at the electron flight tube.24,25

The detached electrons were detected by dual microchannel plates (Hamamatsu,F1552-23S) after flying about 1.2 rn in the flight tube. The optimized magnetic fieldsat the detachment region and in the flight tube were about 800G and 2G, respectively,and the resolution of the photoelectron spectra was about 120 meV for the 1.23 eVpeak of the Cu- ion with a detachment laser of 355 nm. The electron signals werestored in a digital oscilloscope (LeCroy 7200) after amplified by a fast preamplifier(HP, 8447D). Electron counts were accumulated as a function of flight time in theoscilloscope.

Third and fourth harmonics of a Nd:YAG laser (Quanta-Ray, DCR-2A) were usedfor electron detachment with a typical laser fluence of ca. 5 mJ/cm2. Under thedetachment by fourth harmonic (266 nm, 4.66 eV), background electron signals fromthe surface of the chamber cannot be negligible: The background electrons have anenergy distribution peaked around 3.8 eV. Thus the photoelectron spectrum was ob-tained by subtracting the background spectrum from the total spectrum. Thephotoelectron spectra using the third harmonic (355 nm, 3.49 eV) were also recordedfor comparison with those taken by 266-nm detachment.

3. RESULTS AND DISCUSSION

3.1. Negative Ions of the Copper-Water Clusters

Figure 2 shows a typical TOF mass spectrum of the cluster negative ions formed inthe source. These ions were monitored by the microchannel plate placed at the endof the photoelectron energy analyzer chamber without using the deceleration platesand the potential switching grids. Because a Cu atom has a positive electron affinity,1.23 eV,26 stable negative atomic ions and hydrated atomic ions can be produced.Actually, two series of ion signals which can be assigned to those of 6-3Cu-(HzO)nand 65Cu-(HzO)n were observed in the mass spectrum. We can also discern anotherseries of cluster ions, CuOH-(HzO)n_, which are single unit mass smaller than theabove series and thus partly superimposed on the above series. From the knownisotopic abundance of Cu, 63Cu:65Cu 69.2:30.8, the fraction of CuOH- was estimatedto be ca. 30% of that of Cu-(H20). The fractions of larger CuOH-(HzO)n_ clusterions with respect to those of Cu-(HzO)n were almost the same as that for n 1 fromthe shape of the TOF mass peaks, though these ions signals becomes congested eachother with increasing number of water molecules. Beside the two series of cluster


2 4 6

’I

20 30 40 50TIME-OF-FLIGHT

Figure 2 Typical time-of-flight mass spectrum of the negative ions of the copper-water clusters nas-cently produced in the source.

ions noted above, the ions of copper dimer solvated with water, CH2-(HzO)n alsoappear in the mass spectrum. Although the ions of water clusters, (HzO)m- (m n+ 7), have the same masses as 63Cu-(HzO)n, the fractions of the former ions areconsidered to be rather small for m < 10, i.e., n < 3, from the intensity of the isotopicseries of the latter ions in the mass spectrum and also from the previous works onthe mass spectroscopy of the (HzO)m- clusters.27-3

3.2. Assignment of the Bands in the Photoelectron Spectra of Cu-(HzO)n

Photoelectron spectra of Cu-(H20)n for n 0-4 detached at 266 nm are depicted inFig. 3. All of the bands observed in these spectra were reproduced within an errorof 0.10 eV in the spectra obtained by 355-nm detachment. The spectrum of Cu- (Fig.3a) consists of an intense band at 1.23 eV and two weak bands at 2.6 and 2.8 eV:the first band corresponds to the transition of Cu 2S(3d4sl) Cu- tS(dls2), andthe remaining bands to those of Cu 205/z(d9s2 ’- Cu- S and Cu 2D3/2(d9s2 Cu-S, respectively. This spectrum was frequently reported in the study of Cu,- clustersby several groups.31-33 The relative intensities of the 3d- bands at 2.6 and 2.8 eVwith respect to the 4s- band at 1.23 eV are almost the same as those reported byLineberger and his coworkers,3 though they are smaller than those reported byCheshnovsky et al. 32 and Cha et al.33

The spectra for Cu-(HzO)n with n 1 and 2 exhibit more complicated feature thanthat of Cu- as shown in Figs. 3b and 3c. This may be due to the spectral contamina-


Cu-(tl20)n

n= 0 a)

b)

2 * c)

3 o , od)

4 e)

4 3 2 0ELECTRON BINDING ENERGY eV

Cu2-(ll20)n

o g)

4 3 2 0ELECTRON BINDING ENERGY/eV

Figure 3 Photoelectron spectra of Cu-(H20), (n 0-4) and Cu2-(H20)n (n 0 and 1) with detachmentat 266 nm (4.66 eV). Asterisks in b)-d) show the bands from the CuOH-(H20), ions. For the spectra ofCu-(H20)3 (d) and Cu-(HO) (g), the bands from coexisting ions which were not discriminated by pulsedmass gate are shown by circles (see text).

tion by CuOH-(H/O),_. As for n 1, the relative intensity of CuOH- was estimatedto be ca. 30% of Cu-(H20 from the mass spectrum. Since the CuOH-(HzO),n_ ionsare only single unit mass smaller than Cu-(HzO)n, we could not discriminate themby the pulsed mass gate. We also measured the spectra with various timings of thedetachment-laser irradiation after decelerating the ions. However, the features of thespectra did not change substantially. This is because the ions diffuse spatially after


deceleration. On the other hand, the relative intensities of the lowest energy bandsin the spectra for n 1 and 2 (indicated by asterisks in Fig. 3) are found to be muchmore sensitive to the source conditions, especially to the fluence of the vaporizationlaser, than those of the higher energy bands. In the production of the CuOH-(H20),ions, it is expected that extra photons may be needed to dissociate the water molecule.Although the EA value of CuOH- is not reported so far both experimentally andtheoretically, it is considered to be positive because the stable negative ions with alifetime longer than the flight time in the mass spectrometer, several tens of s, wereobserved in the TOF mass spectrum. It is also expected that the EA of CuOH is notso large though it is a positive value from the following reasons. In the neutralCuOH, the covalent bond is expected to be formed between the 4s electron of themetal and the open shell rr electron on the OH, yielding a A’ state in CL, symmetry.Because of the large EA of OH (1.83 eV)26,34,35 and the small ionization potential(IP) of Cu (7.72 eV),36 this bond is expected to be polarized toward the OH. Recentresults on the spectroscopic37,38 and theoretical studies39,4 support this expectation:The ground state (tA’) geometry of the CuOH has a C. symmetry with Cu-O-H angleof 110-111, which is close to that of H20 (105). This result suggests a substantialcontribution of the covalency to the Cu-OH bonding in CuOH. Furthermore, Mullikenpopulation analysis shows that the ionic character of Cu/-OH is also included inthe bonding.4 In contrast, the excess electron in CuOH-is probably in the antibondingorbital and makes the Cu-OH bonding weaker than in the neutral. Therefore the EAof CuOH is expected to be smaller than that of Cu. On the basis of these arguments,we have concluded that the lowest energy peaks observed in Figs. 3b and 3c areascribed to the photoelectron from CuOH- and CuOH-(H20). The rest of the bandsobserved in the photoelectron spectra of Cu-(H20) and Cu-(H20)2 are all assignedto the transitions originating from the Cu 2S +-- Cu- S and Cu 2D <--- Cu-S onesas discussed in the next section.The PES results of Cu-(H20)3 are found to be more complicated than those for

n 1 and 2 as shown in Fig. 3d. The observed spectrum has bands peaked at 0.88,1.71, 2.28, and 2.79 eV. The band at 1.71 eV is attributed to the electron detachmentfrom CuOH-(H20)2 in accordance with the previous discussions. The complexity ofthe spectrum is also due to the incapability of mass selection of Cu-(H20)3 fromother ions in the pulsed mass gate: The detachment signals from the Cu2- ions (m/z

126, 128 and 130) are included in addition to those from Cu-(H20)3 (m/z 117and 119) in this spectrum. Since the photoelectron spectrum of Cu- consists of twobands at 0.92 and 2.73 eV as shown in Fig. 3f, the bands at 0.88 and 2.79 eV observedin the spectrum are ascribed to the detachment from Cu2-. Therefore, the band at2.28 eV can be assigned to the detachment from Cu-(H20)3, which corresponds tothe transition to the neutral cluster state derived from the 2S state of the Cu atom.We could not observe the band originating from the transition to the 2D state,probably due to the large background signal.The spectrum of Cu-(H20)4 exhibits a band peaked at 2.77 eV as shown in Fig.

3e. In contrast to the case for n 1-3, no band assignable to the detachment fromCuOH-(H20)3 is observed. The observed band at 2.77 eV is also assignable to thedetachment from Cu2-(H20), because the band having the similar spectral feature is


also observed in the CU2-(H20 spectrum as shown in Fig. 3g. However, this pos-sibility is excluded because of the missing of the lower energy band at 1.28 eV ofCUz-(HzO in the Cu-(H20)4 spectrum. Therefore, the band at 2.77 eV is believed tobe mostly due to the detachment from Cw(H20)4.

3.3. Hydration of Cu(2S and 2D) and Cu-(1S)

Figure 4 depicts the vertical detachment energies (VDEs), derived from the peakpositions of the photoelectron spectra, for all ions examined here. Lowest energytransitions are attributed to the photoelectron detachment from CuOH-(HzO)n as dis-cussed above. Higher two series of the bands are attributed to the transitions forCu-(HzO)n: The first series of transition shows the lowest VDEs of the electron whichoriginates from the 4s orbital of the Cu- in Cu-(HzO)n. The VDEs were found toincrease monotonically with increasing number of water molecules: The averagestabilization energy with an increase of one water molecule is determined to be ca.0.38 eV from this figure. It is also noted that the bandwidths of the series are almostconstant (=300 meV) up to n 3. These results seem to imply that the excess electronin the cluster is localized on the Cu atom at least up to n 4 in the ground state,and the successive solvation by water results simply in the stabilization of the states.This conclusion is consistent with the fact that the EA of the Cu atom is positive(1.23 eV)34,35 whereas those of the water clusters, (HzO)n with n < 6 are negativeexcept for n 2 (< 0.1 eV) from the PES study of (HzO)n-.41 We have also observed

"-- 3

’i ’1 ’!

Cu-(H20)n

,!,

0 2 3 4

Cu2-(H20)n

NUMBER OF WATER MOLECUI.ES

Figure 4 Vertical detachment energies (VDEs) of the electron for the neutral ground (open circles)and the excited states (filled circles) as a function of the number of solvent water molecules for Cu-and Cu2-. The VDEs for CuOH-(HzO)n_ are also shown by squares.


the secondarily lowest VDEs which originate from the 3d electron of Cu-up ton 2. Though the 3d electron detachment from Cu- produces two spin-orbit statesof Cu, 2D5/2 and 2D3/2, as seen in Fig. 3a, we cannot discern these states for n 1and 2 because of the broadening of the band. This is probably due to the structuraldifference between the excited states and the negative ions and also due to the con-gestion of the vibrational modes. This series also shows a linear dependence uponthe stepwise hydration. The energy increase for this series is determined to be ca.0.2 eV per water molecule.From the above VDEs of Cu-(HzO)n, we estimated the successive hydration ener-

gies for the anion states and the 2D excited states. Unfortunately, there is noexperimental and theoretical data on the hydration energy of the neutral Cu(HzO)n,except for that of Cu(H20 calculated by Bauschlicher.42 Thus we adopted thetheoretical result (0.2 eV). According to his calculations, the bonding between themetal and a water molecule involves the electrostatic interaction arising from thepenetration of the ligand dipole into the metal charge cloud. This bonding is enhancedby the polarization of the 4s away from the ligand lone pair and by the polarizationof the ligand. We also tentatively assumed that the hydration energies for Cu(HzO)nare the same with that for Cu. The results are summarized in Fig. 5.As for the 2D excited state of Cu, the successive hydration energy is found to be

ca. 0.3 eV, which is slightly larger than that of the neutral ground state. The differencein hydration energy for these states is qualitatively explained as follows. Because ofthe electronic configuration of 3d94s in the 2D state of Cu, it seems that the repulsiveinteraction between the diffuse 4s electrons and the ligand lone pair may be larger

Cu-W4

Figure 5 Energy diagram of the neutral Cu(H20)n and the Cu-(H20)n clusters ions. Water moleculesare designated by W. All values are shown in eV.


than that in the ground state in which one electron is in the compact 3d orbital.However, when the electron density of the 3dr orbital is reduced in the excited state,binding energy between the metal and the ligand may increase as a result of reducednuclear shielding for the H20 lone pair electrons due to the one less electron alongthe internuclear axis.The hydration energies of the Cu-(HzO)n, n 0-4, are found to be almost constant,

-0.6 eV, as shown in Fig. 5. These results indicate that the first solvation shell aroundthe Cu- ion is not still filled at n 3. It is informative to compare these results withthose of the hydrated halide negative ions, X-(HzO)n (X C1, Br and I),2’21’43-45 whichwere the only hydrated atomic negative ions investigated extensively by both ex-perimentally and theoretically. The successive hydration energies were determinedby high-pressure mass spectrometry43,44 and recently by photoelectron spectros-copy.2’21 According to the results, the binding energies of the halide ions with awater molecule were determined to be 0.5743 and 0.6520,44 for CI-, 0.5543 and 0.5720

for Br-, and 0.44,43 0.48,44 and 0.45 eV for I-, respectively. The simplest view ofthe interaction between X- and H20 would be the electrostatic one between the excesselectron on the halogen atom and the dipole of the ligand. The energy of this inter-action is inversely proportional to the distance between the halide ion and the dipole.In fact, the crystal ionic radii of the ions, 1.81 for CI-, 1.96 for Br-, and 2.20 A forI-,46 are qualitatively consistent with the order of the determined hydration energiesfor the three halide negative ions. As for the Cu- ion, ionic radius has not beenreported so far. However, assuming a correlation between the ionic radii and the vander Waals radii, the ionic radius of Cu- was slightly smaller than the above threehalide negative ions.v The hydration energy of ca. 0.6 eV for Cu- is qualitativelyexplained by this simple electrostatic consideration.The equilibrium structures and the hydration energies for the hydrated halide ions

are also predicted theoretically? The calculated successive hydration energies are ingood agreement with those determined experimentally.2,2,3,44 According to thesestudies, hydrogen bonding has found to contribute considerably to the interactionbetween the ion and the water molecule in addition to the electrostatic interactionnoted above. In the optimized structure for X-(H20), the ion interacts most stronglywith one of the hydrogen atoms of H20 to form a hydrogen bonding, and as a result,the complex has an asymmetric structure of H20 relative to X-. For larger clusters,in addition to the interaction between the ions and the ligands, the ligand watermolecules are also held together by networks of hydrogen bonding. The importanceof the hydrogen bonding was also pointed out in the theoretical study on H-(HzO)n:48The optimal structure calculated for n 1 is similar to that for X-(H20), and thosefor n 2 and 3 have hydrogen bondings not only between the H- ion and the ligandsbut also between the ligand pairs, as in the cases of hydrated halide ions. In thehydrated halide negative ions, the successive hydration energies for n 2-5 werefound to be almost constant. This fact indicates that the formation of hydrogen bond-ings between the ligands has a substantial contribution in the hydration energies.Since the Cu- ion has the closed shell configuration as in the cases of the X- andH- ions, the hydrated Cu- ions are expected to have the similar geometrical structuresto those for the latter ions. Therefore, the hydrogen bondings between the metal ion


and the ligand molecules as well as between the ligand molecules may play animportant role in the metal-ligand interaction in addition to the electrostatic interac-tion. This may be the reason for the observation that the successive hydration energiesfor Cu-(H20)n are almost constant up to n 4, as noted above in the case of hydratedhalide ions.

3.4. Hydration to the Copper Dimer and Its Negative Ion

The photoelectron spectra of negative ions of copper dimer and its hydrate are shownin Figs. 3f and 3g. The spectrum of Cu2- is almost consistent with those reportedpreviously by other authors:31-33 The first and second bands peaked at 0.88 and 2.79eV correspond to the transitions from Cu2- XZEu to the neutral ground state, Cu2XIEg+, and the triplet excited state, Cu2 a3]u+, respectively. The spectrum of Cu2-(H20) exhibits two peaks at 1.28 and 2.82 eV with a shoulder at 3.21 eV. As men-tioned previously, the Cu-(H20)4 ion could not be discriminated from Cu2-(H20) bythe pulsed mass gate, and as a result, the spectra may contain the bands attributedto the former ion. In fact, the spectral feature of the second band is almost the sameas those of Cu-(H20)4, and thus we concluded that this band is ascribed to the Cu-(H20)4 ion. Therefore, the first band at 1.28 eV can be assigned to the transition tothe neutral ground state, and the shoulder at 3.21 eV may correspond to the transitionto the excited state. The VDEs of Cu2- and Cu2-(H20) are plotted in Fig. 4. Boththe first and the second bands are stabilized ca. 0.4 eV by hydration, which is slightlylarger than the stabilization for the first band for Cu- (0.35 eV). As for the first band,this difference is expected to be mainly due to that of the hydration energies for thenegative ions: The hydration energy for Cu2- is larger than that for Cu- because ofthe polarization of the dimer anion.

4. CONCLUDING REMARKS

The technique of the negative-ion photoelectron spectroscopy has been applied forthe first time to the hydrated metal atom clusters. We have measured thephotoelectron spectra of Cu-(H20)n (n 0-4) and Cuz-(H20)n (n 0 and 1). Thelowest bands in the spectra of Cu-(H20), (n 1-3) are found to be assignable tothe detached electrons from the CuOH-(H20),_ ions, and the remaining bands areascribed to the transitions to the neutral states derived from the ground 2S(3d4s)and the excited 2D(3d94s2) states of the free Cu atom. Vertical detachment energiesdetermined from the peak positions of the bands in the spectra show linear depend-ence on the number of water molecules both for the first and second bands. Theenergy diagram including the negative ion states and the ground and the excitedstates of the neutrals is constructed for n 0-4. The successive hydration energiesfor Cu-(HO), are found to be almost constant at ca. 0.6 eV from the energy diagram.The hydrogen bonding is expected to have a substantial contribution in the hydrationenergies, in addition to the electrostatic interaction. As for Cu2-, the hydration energyis found to be slightly larger than that for Cu- because of the polarization.


Acknowledgments

The authors wish to thank the members of the Equipment Development Center ofInstitute for Molecular Science for their contributions in constructing the experimen-tal apparatus. We also thank. Dr. A. Nakajima for giving valuable advice fordeveloping the magnetic-bottle photoelectron spectrometer. The present work hasbeen partly supported by a Grant-in-Aid for Scientific Research from the Ministryof Education, Science, and Culture.

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